Study On Synchronization For OFDM System In Burst Mode

As a promising candidate technology of the physical layer for future broadband high-data-rate wireless communications, Orthogonal Frequency Division Multiplexing (OFDM) has invoked a popular research interest in recent year. Appling OFDM in burst mode transmission is inevitable selection of data communication including Internet. Synchronization is the most important task that any receiver must complete. Based on the synchronization, the other modern communication techniques, including multi-antennas, space-time code, adaptive coded modulation, etc., can be applied. To burst OFDM system, synchronization is especially important. The existence of carrier frequency offset (CFO) disable sub-carrier to maintain orthogonality, and symbol timing deviation can further arise inter-symbol interference. All of synchronization errors seriously degrade the system performance, which hamper OFDM to manifest its advantages.We apply the basic digital signal processing knowledge, statistical signal estimation and detection theory, attempt to propose a set of low-complexly and effective synchronization strategy for burst OFDM receiver. So performance and complexity are cared throughout this dissertation. Synchronization is discussed in 2nd, 3rd, 4th chapters. Some fruits in channel estimation and channel decoding are put together into the 5th chapter, which enable this dissertation to constitute a complete receiver system.We point out the strategy of using the preamble to fast and accurate acquire signal in the limited time and using pilot to track residual synchronization error after acquisition is suitable for burst OFDM receiver. A low-complex automatic gain control (AGC) adjustment strategy by threshold comparison is proposed. A low-complexity CFAR (const false-alarm rate) frame detector is proposed, which exploits periodic structure of preamble and outperforms conventional autocorrelation detector. A ML integer-CFO estimator exploiting statistical characteristics of frequency selective channel is proposed. When channel covariance is unavailable, two simplified algorithms, one utilizing pilot energy and the other named double-correlation, are derived. The performance of synchronization tracking using pilot is analyzed, followed by a numerical method to determine the optimal lock loop parameters to minimize average bit error rate (BER). Comparative analysis of channel estimate algorithms utilizing preamble are conducted. Following is an analytical expression to calculate the BER of M-QAM under Rayleigh channel when channel estimation error exists. Explanation of simplified soft bit metric-logarithm likelihood ratio (LLR) of M-qam is done through graph and then simplified LLR expression for p/8-8PSK and p/16-16PSK is proposed. Finally, the performance of complete receiver, including synchronization, tracking, channel estimation and channel decoding, is given by computer simulation.